The present invention generally relates to the temperature monitoring of a resistance furnace and, in particular, relates to such an apparatus which is independent of any current leakage through the walls of such a furnace.
Many modern analytical instruments utilize a resistance furnace to provide precisely controlled temperatures to elements under test. As one element of the temperature control mechanism for such a furnace, a thermocouple is generally employed in conjunction with the furnace to provide a feedback signal to maintain the furnace at a particular temperature or to control the time rate of change of temperature of the furnace. In a sensitive thermal instrument, such as a differential thermal analyzer, the effect of current leakage through the walls of the furnace into the electrical measuring circuit can create intolerable errors in the particular measurement being made.
In general, the furnace segment includes a wire coil imbedded in a ceramic material such as aluminum oxide (Al.sub.2 O.sub.3). While such ceramic materials are substantially thermally inert over a wide temperature range, the electrical characteristics are such that significant leakage current can be conducted therethrough. It is a known characteristic of such materials that current leakage therethrough increases as the temperature increases so that at temperatures above 1500.degree. C. the current leakage is unacceptably high. For example, in a ten amp, 60 Hz furnace, the current leakage can generate a spurious voltage in the thermocouple amplifier circuit. Since the accuracy requirements of the temperatures involved are quite stringent, i.e. sensitivities on the order of about 0.05.degree. C. in differential thermal analyzers the spurious voltage caused by leakage current can produce significant errors.
One solution to this problem is to position the thermocouple a short distance, i.e. spaced apart, from the external wall of the furnace, leaving an air gap therebetween. While this ensures that no current is leaked into the measuring circuit, and hence no voltage drop created, from the furnace itself, the separation of the thermocouple from the furnace wall consequently reduces the accuracy of the thermocouple reading due to the temperature gradient created across the air gap. Another basic drawback of an air gap is that an additional transport lag is introduced into the temperature control feedback loop, thus requiring a reduction in the loop gain in order to maintain loop stability. Such a reduction in loop gain degrades the temperature control sensitivity in the circuit.